The rapid transformation of Brazilian society from a rural into an urban population in a short period of time may have influenced fertility developments in the country. This study analyses cohort fertility differentials according to the migration status of women born between 1921 and 1945. Using census data from 1970 and 1980, we reconstruct cohort fertility at the beginning of the Fertility Transition for two regions, Southeast and Northeast. We use decomposition to quantify the effect of migration on fertility by decomposing the overall change in the CFRs into migration composition effect and rate effects. The differential between migrants and non-migrants and the fertility levels differs remarkably between NE and SE, but fertility declined among both, non-migrants and migrants. In the case of the Northeast, a less urbanised region, the fertility differentials between the groups of migrants were smaller, while in the Southeast, urbanisation already reached lower rates at the beginning of the transition, the differentials are more pronounced. The decomposition shows that the decline in cohort in CFRs being caused mostly by changes in fertility behaviours, but migration significantly contributed to the fertility decline of the 1931-1935 birth cohort.